1. Field of the Invention
The invention relates generally to signal receivers and, more particularly, to receivers that distinguish between direct and multipath components of a received signal.
2. Background Information
Various types of receivers, such as GPS receivers, which are used for receiving coded electromagnetic signals emitted by one or more transmitters, are well known. As shown in FIG. 1, the received signals typically include both a line of sight, or direct, signal 21 and one or more signals 23 that are reflected by a surface 26 of an object 25. The object can be a terrestrial structure, various other stationary structures, moving vehicles and the like. Note that while only one reflected signal is shown for clarity, the following discussion applies equally well to situations in which additional reflected signals are present. The direct signal 21 and the reflected signal 23 are incident upon a receiver antenna 11 and are transmitted to a receiver 24.
The receiver 24 cannot distinguish between the direct signal, and the reflected signal that arrives at a slightly later time. The receiver processes the combination of the direct signal and the reflected signal as if it were an undistorted signal, and thus, errors are introduced. The errors are commonly referred to as multipath errors, and the reflected signals are commonly referred to as multipath signals. Multipath signals that are delayed by two or less chips of a PRN code that is included in the direct signal are of particular interest, because a locally-produced PRN code at the receiver partially correlates to these signals.
One method of avoiding the multipath signals is to use a highly-directional antenna that is precisely oriented to receive the direct signal and not the multipath signals. However, if the desired signal originates in a moving transmitter, such as a satellite 27, use of a directional antenna is not practical. In such cases, is becomes desirable to locate the receiver antenna 11 in a location away from the surfaces 26 that produce the multipath signals 23.
Conventionally, a location for the receiver antenna 11 is selected by visually inspecting the proposed physical antenna site and placing the antenna at a location that is perceived to contain the fewest sources of multipath signals. As can be appreciated, such a visual assessment is qualitative and will not always produce reliable or consistent results. What is needed is a mechanism for making a quantitative assessment of possible locations. Further, for existing locations, what is needed is a mechanism for quantitatively assessing signal quality in environments in which multipath signals may be intermittently introduced, such as when objects move onto or off of the antenna site or nearby environs, or when the signals received from satellites in particular orientations are more adversely affected by the multipath signals.
Another source of error in signal correlation is satellite failure. A satellite is operating in a failure mode when it produces anomalous signals, such as in which code chip transitions induce ringing, the transitions lead or lag expected transition times, and so forth. The receiver processes these signals in the usual manner and produces correlation functions that are distorted, such as functions with flat or multiple correlation peaks. The receiver then attempts to track the received signal based on the distorted correlation functions, and substantial tracking errors may occur. Presently, satellite failures are detected by analyzing the correlation functions over time. The satellite failures are difficult to detect in an environment that includes certain multipath sources, however, because of the inherent distortion of the correlation function that is attributable to the multipath signals. Accordingly, what is needed is a mechanism that quantitatively determines when the received signal is distorted in a manner that is associated with substantial tracking errors.
The invention is a multipath meter that analyses estimates of parameters associated with a direct signal component and one or more multipath signal components of a signal received by an antenna. The meter analyses the parameter estimates in order to facilitate the select of a location for an antenna, monitor signal quality at an existing antenna site, and/or to determine if satellite failure or some other type of signal failure has occurred.
In brief summary, the multipath meter makes available to a user or for analysis information related to the contributions of the direct signal and the multipath signals to the received signal. The information includes estimates of various parameters such as delay, relative amplitude and phase of the direct and multipath signals. The meter calculates a ratio of the amplitudes of the direct and the multipath signals, to determine the severity of the multipath signals and, thus, the signal quality. The same ratio values may also be used to determine the advisability of the associated location as an antenna site. The meter further calculates correlator residual values, as discussed in more detail below, to determine how well the receiver compensates for the multipath signals. The meter can then determine if the site and/or signal quality corresponds to a condition in which xe2x80x9cexcessivexe2x80x9d multipath signals are introduced into the received signal, that is, a condition that is associated with rather substantial tracking errors.
The meter also calculates normalized error ratios and normalized error values for selected correlators used by the receiver to produce the estimates of the signal parameters. Based on these ratios and values, and the determination of signal quality the meter determines if a satellite failure or other signal failure has occurred, as discussed in more detail below.
The meter makes its calculations in real time for signal quality monitoring and signal failure detection. The meter also retains the calculated values, or certain of the values, and plots the values over time so that a user can review them to determine the desirability of a particular location as an antenna site.